@syb tapping into the wiring harness sounds to complicated and cumbersome. Go for either the backup light switch, a junction connector or even the fuse box. Looks like the backup light is fused by itself.
The back-up light switch is located on top of trans, rear towards the firewall,The Fuse located in the under hood fuse/relay box. Take a test light and with key turned to the on position. Ground the clip end of the test light and use the pointed end to touch each wire connector. One wire on the switch should be hot (B+ test light turns on), this means you have voltage to the switch. Then with the wires connected to the switch, put trans in reverse. With key in the on position test the other wire which will also become hot (B+ test light turns on). That wire would be a good candidate to tap into it for your backup camera.